The field of the invention relates to organic light-emitting devices with improved electrodes and their deposition.
Organic light-emitting devices (OLEDs) such as described in U.S. Pat. No. 5,247,190 assigned to Cambridge Display Technology Limited or in Van Slyke et al, U.S. Pat. No. 4,539,507, the contents of which are herein incorporated by reference and example, have great potential for use in various display applications, either as monochrome or multi-colour displays. Principally, an OLED consists of an anode that injects positive charge carriers, a cathode that injects negative charge carriers and at least one organic electroluminescent layer sandwiched between the two electrodes. Typically although not necessarily, the anode would be a thin film of, for example, indium-tin-oxide (ITO), a semi-transparent conductive oxide which is commercially readily available already deposited on glass or plastic substrates. The organic layer(s) would then normally be deposited onto the ITO-coated substrate by, for example, evaporation/sublimation or spin-coating, blade-coating, dip-coating or meniscus-coating. The final step of depositing the cathode layer onto the top organic layer is normally performed by thermal evaporation or sputtering of a suitable cathode metal in vacuum. Layers of Al, Ca or alloys of Mg:Ag or Mg:In, or Al alloys are often used as cathode materials.
One of the key advantages of the OLED technology is that devices can be operated at low drive voltages, provided that suitable electroluminescent organic layers and electrodes with good efficiencies for the injection of positive and negative charge are used. In order to achieve good performance in OLEDs it is of great importance to optimise all the individual layers, the anode, the cathode and the organic layer(s), as well as the interfaces between the layers.
A cathode of high quality is of great importance to achieve overall high performance in OLEDs, judged on criteria such as power efficiency, low drive voltage, shelf life, operating life, and stability in stringent environmental conditions such as high temperature and/or high humidity, etc. The criteria for the quality of the cathode are in particular but not exclusively the work function, corrosion resistance, morphology and barrier properties, adhesion to the polymer and sheet resistance.
Metallic cathode layers for OLEDs are most commonly deposited by simple thermal evaporation of the cathode material in vacuum. Similarly, cathode layers consisting of a metal alloy can be deposited by thermal evaporation from two or more sources containing the alloy constituents and by choosing appropriate relative depositing rates to achieve the desired relative alloy composition.
However, simple thermal evaporation of metals onto OLEDs to form a cathode layer can result in poor adhesion between the cathode and the top organic layer and, very often, the morphology of the evaporated layer is polycrystalline with large average grain size such that there is a high density of grain-boundary for diffusion of ambient gases such as oxygen and moisture into the device. Poor adhesion and large grain-size polycrystalline morphology can severely deteriorate the OLED performance, in particular environmental stability (device shelf-life and operating life, corrosion of the cathode).
The same issues (adhesion, morphology) apply to the case in which an OLED is built up from the cathode, i.e. when the cathode is deposited on the substrate with the subsequent deposition of the organic layer(s) and as the final step deposition of the anode on top of the top organic layer.
Simple thermal evaporation from different sources of elemental metal or evaporation of a ready-made alloy to obtain cathodes in alloy form also has problems. For example, if a cathode alloy layer comprising reactive low work function elements, such as alkali or earth-alkali metals, is required the processing and/or handling of these elements in a normal environment in air may be difficult if not impossible. Alternatively, if an alloy itself is evaporated, the alloy composition of the deposit (cathode) may be difficult to control due to, for example, different thermal properties and differential evaporation rates of the source-alloy constituents.
It is thus an object of the present invention to provide a structure and method of fabrication for an organic electroluminescent device that overcomes, or at least minimises, the above-described problems.
According to a first aspect of the present invention there is provided an organic light-emitting device, comprising at least one layer of a light-emissive organic material arranged between first and second electrodes, wherein at least one of the electrodes is a multi-layered structure, each layer of the multi-layered structure being a DC magnetron sputtered layer.
This first aspect of the invention also provides a method of fabricating an organic light-emitting device, comprising the steps of:
forming an electrode over a substrate;
forming at least one layer of a light emissive organic material over the electrode; and
forming a multi-layered electrode structure over the at least one layer of organic material, each layer of the multi-layered structure being a DC magnetron sputtered layer.
This first aspect of the invention further provides a method of fabricating an organic light-emitting device, comprising the steps of:
forming a multi-layered electrode structure over a substrate, each layer of the multilayered structure being a DC magnetron sputtered layer;
forming at least one layer of a light-emissive organic material over the multi-layer ed electrode ; and
forming an electrode over the at least one layer of organic material.
According to a second aspect of the present invention there is provided an organic light-emitting device, comprising:
a first electrode;
two or more layers of light-emissive organic material formed over the first electrode, wherein the uppermost layer of organic material is more resistant to sputter deposition than the underlying layer of organic material; and
a second electrode formed over the uppermost layer of organic material, wherein the second electrode comprises at least one layer, the layer adjacent the uppermost layer of organic material being a sputtered layer.
This second aspect of the invention also provides n organic light-emitting device, wherein-the uppermost layer of organic material is substantially resistant to sputter deposition.
Thus, the first and second aspects of the invention provide an organic electroluminescent device with a metallic cathode of compact morphology with low average grain size and good adhesion to the adjacent layer of the OLED stack with the said cathode laid down by sputter deposition. Good adhesion between the cathode and the adjacent layer minimises delamination and the ingress of, for example, oxygen, moisture, solvents or other low molecular weight compounds at/along said interface. Also, the compact morphology of the cathode metal layer minimises diffusion of ambient species such as oxygen, moisture, solvents or other low molecular weight compounds into the OLED through the cathode layer itself. Said cathode forms the electron injecting electrode for an OLED with at least one electroluminescent organic layer between said cathode and an anode that injects positive charge carriers. The organic electroluminescent layer(s) are preferably but not necessarily conjugated polymers.
In the first aspect of the invention the sputter deposition process is DC magnetron sputtering. In the second aspect of the invention the light-emissive organic material comprises two or more layers, the uppermost layer being more sputter resistant to sputter deposition than the underlying layer of organic material.